The ants may move the insects to areas on the plants with the best sap.

When it rains they may move them to sheltered places, even sometimes into their own nests.

Although this process seems very plesant for both parties, recent studies show that ants sometimes clip the wings off aphids to stop them flying away. They also use chemicals found on their feet to drug them, preventing their wings from developing. It appears that the ants are very much in charge. (Telegraph)